I understand what you mean. Though my collisions are done mariokart style(arcade like) and do not hamper the gameplay(or block paths) with the expection of a bounce away from the opposing kart. If this game was very dependant on realistic collisions then this would be a problem. Either way the server still contains the "real" positions of all entities at all times. If you keep the world on the client static to the entity(meaning, things dont affect movement on your comp any different than they do on others) you should be fine. I'd use the server positions and some distance testing to see if there is a collision then initiate the bounce sequence on the clients. So lets say theres a thing on the ground that you run into and spins you out. You'd have the server do the scanning for karts. That way it's fair. But your right about the potential issues... I think i'll have to look into this a bit further yet. Though in the meantime i'll keep to this method and will soon see how it all pans out when I do some more indepth testing(crosses fingers).